1. Field of the Invention
This invention relates to a master cylinder for generating a pressure.
2. Description of the Related Art
A conventional master cylinder as prior art is disclosed in Japanese Utility Model Laid Open No. 63(1988)-176864. FIG. 4 is a partially enlarged sectional view of the conventional master cylinder shown in the prior art. In accordance with FIG. 4, the master cylinder comprises a cylinder body 100 including a cylinder portion 100a and a piston 105 including a seal member 101. The piston 105 is fluid-tightly fit in the cylinder portion 100a via the seal member 101 so as to establish a pressure chamber 102 connected to wheel cylinders and a supply chamber 103 connected to a reservoir. The piston 105 further includes a connecting passage 104 connecting the pressure chamber 102 with the supply chamber 103. The master cylinder further comprises a return spring 106 biassing the piston 105 to a rest position, a valve mechanism V and a restriction member 108 having an orifice 107 which is installed in the connecting passage 104. The valve mechanism V allows connection between the pressure chamber 102 and the supply chamber 103 when the piston 105 is located at the rest position. To the contrary, the valve mechanism V disconnects the pressure chamber 102 from the supply chamber 103 when the piston 105 moves for a predetermined distance.
In accordance with the above conventional master cylinder, a pressure is generated in the pressure chamber 102 before the valve mechanism V closes the connecting passage 104 since the restriction member 108 restricts a quantity of fluid flowed into the supply chamber 103 from the pressure chamber 102. Therefore, this conventional structure may improve the operation feeling at the time when the master cylinder operation is initiated.
However, because the restriction member 108 restricts a quantity of the fluid returning to the pressure chamber 102 from the supply chamber 103, the fluid flowing into the pressure chamber 102 via the connecting passage 104 is limited when the piston 105 is released. Therefore, since a negative pressure generated in the pressure chamber 102 continues to exist for a long time, the sealing force of the seal member 101 must be increased. Furthermore, it is difficult to shorten the brake pedal stroke when the brake pedal operation and the brake pedal release are alternately repeated (called a "double brake" when the master cylinder is applied to a brake system).